'Species' is a model, it is taxonomists who, quite sensibly, try to find some order in the biomass. Nature didn't invent species, man did. People typically picture 'a species' as a truly discreet unit, I know simple definitions such as the ability to interbreed are often used as explained above; this is fine for generalising. Life and nature is more fuzzy than we would like the 'ring species' are a good example http://en.wikipedia.org/wiki/Ring_species
I think, I may be wrong, but in the same fashion as the species changes along a distance transect for the ring species, a temporal transect would show a similar pattern. By this I mean a sequence of organisms with very little difference between them spread out over time. When people talk of 'missing links; in fossil records or similar I think the confusion is based around the same misunderstanding.

We talk of visible light, IR, X-ray - these are 'species' in a sense. The EM spectrum is exactly that, a continuum. Scientist have spit it up into more convenient units, the cut off points between different types of EM radiation may be in places for good reason, but they aren't truly discreet since there is continuity between them.

DNA analysis is making taxonomists move the odd organism across entire groups, convergent evolution makes it difficult to assess a species on looks and habits alone. It also seems that there can be bigger (%)genetic differences within a population of a certain species than between some species.

I also see the concept of 'race' in a similar fashion. I don't really follow it so well as once again the lines between who is and isn't in a certain group aren't exactly scientific. Maybe we could make assessments based on Neanderthal/Denisovian DNA or lack of in a human? Least it would be less subjective.

That's because neither lions nor tigers have any reproductive isolation mechanisms.
One question: Why the heck would a lion and a tiger mate? Edit: because of artificial selection. Although, how do you suppose a human could benefit from creating a lion-tiger hybrid? Other than to say "Look, I crossed two animal species!"

Although, how do you suppose a human could benefit from creating a lion-tiger hybrid? Other than to say "Look, I crossed two animal species!"

Off the top of my head increasing the knowledge and skills we have in hybridising animals may help through the development of humanised xenotransplants though that field was pretty much killed off just before the turn of the millennium due to worries over zoonosis.

There is a new, emerging field called dna barcoding. This allows us to catalogue all living species just as we do any merchandise in a store. Each living species (plants, animals, fungi, etc.) has a region of their genome which is unique, the 'dna barcode', and this field is rapidly growing with more and more species being catalogued every day.

This interesting field has actually made some stark discoveries which will affect many areas of our lives moving forward. For example, many species which were once thought to be the same species were identified as actually being 2 different species (these are called cryptic species, they appear identical, but they are indeed different species).

Another interesting implication for example is the discovery of false ingredients in restaurant and packaged food. Mislabelled fish in sushi for example, or false ingredients in herbal teas. Recently, the FDA started using dna barcoding in regulating the importation of fish.

This technique could theoretically be employed to detect a new spcies, however I doubt we'll ever witness a species evolve. According to the theory, the changes which need to occur accumulate at a very very slow pace and speciation would typically take millions of years. And that's according to the theory, which, after all, is just that.

This technique could theoretically be employed to detect a new spcies, however I doubt we'll ever witness a species evolve. According to the theory, the changes which need to occur accumulate at a very very slow pace and speciation would typically take millions of years. And that's according to the theory, which, after all, is just that.

You need to learn what a scientific theory is and also learn about what we have and haven't observed regarding speciation. It might help to read a topic before you wade in, would that you would have noticed someone above you linking to witnessed speciation......

Edit: Note for the OP; it's not a matter of whether you can force to 'species' to hybridize. Whether they breed or don't isn't determined by man's meddling, rather by how they interact in the wild. Reproductive barriers (searchable term) keep two gene pools distinct from one another (good example see the Hawthorn flies). The essence of speciation is reproductive isolation (again searchable term).